1. Field of the Invention
The present invention relates to a magnetooptical recording medium provided with a recording layer composed of a magnetic film allowing information recording by irradiation with a light beam, and a recording method utilizing said recording medium.
2. Description of the Related Background Art
Optical memory devices utilizing laser beam are being actively developed in recent years as useful memories of high density and large capacity. In particular, magnetooptical recording is attracting attention as a rewritable recording method, and the magnetooptical recording media employed as a rewritable optical memory device.
FIG. 1 schematically illustrates a conventional apparatus for such magnetooptical recording, wherein a disk-shaped magnetooptical recording medium or magnetooptical disk 31, provided with a magnetic layer having an easy axis of magnetization perpendicular to the surface of said layer, is rotated by a spindle motor 32. An optical head 34, provided with a laser unit, an objective lens 33 etc., performs information recording by projecting a light beam 35 (turned on and off according to the information to be recorded) onto the disk 31 through the objective lens 33. A bias magnetic field is applied by an electromagnet 36 to an area of the disk 31 irradiated by the light beam. The optical head 34 is moved in the radial direction of the disk 31, thereby recording information in spiral or concentric patterns.
In conventional apparatus as shown in FIG. 1, the information recording and erasure are conducted in steps 30a to 30f shown in FIG. 2. At first, as shown in 30a, the magnetic layer 37, constituting the recording layer of the magnetooptical disk, is magnetized in a predetermined direction. Then, as shown in 30b, the magnetic layer 37 is irradiated by the light beam 35. The irradiated area is heated close to the Curie point of said magnetic layer 37 by absorption of the irradiating beam, thus causing a decrease in the coercive force. In this state the magnet 36 shown in FIG. 1 applies a bias magnetic field B' of a direction opposite to the aforementioned predetermined direction, whereby the magnetization in the area irradiated by the light beam is inverted. Thus, after having passed the position of irradiation, as shown by 30c, a record bit 38 having a direction of magnetization different from that in the surrounding area is formed. The information is recorded as a train of such record bits 38 or an information track.
For erasing the information recorded as in 30d, an unmodulated light beam 35 is projected while a bias magnetic field -B' of a direction opposite to that of the magnetic field at the recording is applied by the magnet 36 as shown in 30e, thereby heating the magnetic layer 37 again to a temperature close to the Curie point. Thus the magnetic layer 37 restores the magnetization aligned in the predetermined direction, thus returning to the state prior to the recording as shown in 30f.
The recorded information can also be reproduced by irradiating the magnetic layer 37 having record bits 38, with an unmodulated light beam of a reduced intensity insufficient for heating to the Curie point, and detecting the direction of polarization of the reflected or transmitted light beam by a known method utilizing magnetooptical effect.
However, in case of rewriting already recorded information, the conventional apparatus as explained above is incapable of so-called overwriting but requires a step of erasing followed by a step of new recording. Thus, in case of changing the information recorded in a track on a magnetooptical disk, it becomes necessary to erase the information of said track in a turn of the disk and to record the new information in a succeeding turn, and such operation inevitably results in a loss of recording speed.
In order to resolve such drawback there has already been proposed an apparatus equipped with a record/reproducing head and a separate erasing head, or an apparatus in which the recording is achieved by modulating the applied magnetic field while a continuous laser beam is projected, but such apparatus are associated with other drawbacks such as being bulky and expensive or incapable of high speed modulation.
On the other hand, in order to improve the recording sensitivity and the S/N ratio at reproduction in such magnetooptical recording medium, technology utilizing two mutually exchange-coupled magnetic layers is disclosed in the Japanese Patent laid-open No. 78652/1982, corresponding to the U.S. patent application Ser. No. 315,467 which is continued as continuation-in-part No. 644,143, which is further continued as Continuation No. 908,934 now U.S. Pat. No. 4,799,114, issued Jan. 17, 1989. In addition to the above-mentioned patent applications, such magnetic layer of two-layered structure was described in "Magnetization Process of Exchange-coupled Ferrimagnetic Double-Layered Films", Kobayashi et al., Japanese Journal of Applied Physics, Vol. 20. No. 11, November 1981, P. 2089-2095 and "Thermomagnetic Writing on Exchange-coupled Amorphous Rare-Earth Iron Double-layer Films" Tsunashima et al., IEEE Transactions on Magnetics, Vol. MAG-17, No. 6, November 1981, P. 2940-2842.
However such exchange-coupled double-layered films are still incapable of overwriting and thus require an erasing step.